1. Field of the Invention
The present invention relates to processes for secondary treatment of wastewater with activated sludge and particularly to such biological wastewater treating systems designed for removal of nutrients, particularly phosphates, in addition to BOD.
2. Prior Art
The activated sludge process has been used for many years for the removal of biological oxygen demand, BOD, from wastewater. The process consists of maintaining an aeration basin in which wastewater is fed to a suspension of microorganisms to form a mixed liquor. The mixed liquor is aerated to furnish oxygen for the respiration of the biomass, which sorbs, assimilates, and metabolizes the biological oxygen demand of the wastewater.
After a suitable period of aeration, the mixed liquor is introduced to a clarifier in which the biomass settles and the treated wastewater overflows into the receiving stream. A major portion of the settled biomass, which is concentrated at the bottom of the clarifier, is recycled to the aeration basin and a minor portion is purged in order to maintain a constant biosolids inventory within the system. This process has been extensively described in the literature and several of its modifications summarized in a special report on "Wastewater Treatment" by R. H. Marks contained in the June 1967 issue of POWER.
A number of techniques have been proposed in more recent years aimed at modification of the conventional activated sludge process to effect or improve removal of nutrients such as nitrogen and/or phosphorus values. Among the various methods disclosed in the prior art for effecting this purpose is that described in U.S. Pat. No. 4,056,465 which in addition to effecting removal of phosphorus and/or nitrogen values from the influent wastewater operates under designed conditions such as to obtain a non-bulking biomass population of highly active, dense and rapidly settling sludge.
In accordance with the process described in the '465 patent the influent wastewater, generally after primary clarification, initially is mixed with recycled sludge in an anaerobic zone and the mixed liquor then is passed to an oxic zone where it is subjected to oxidizing treatment. The oxidized mixed liquor subsequently is passed to a settling zone or clarifier and a portion of the thus activated settled biomass is recycled to the initial anaerobic zone for admixutre with the wastewater influent. The process of the patent is particularly applicable in the treatment of phosphate-containing wastewater. By the initial treatment of the mixed liquor (recycle sludge plus influent wastewater) under anaerobic conditions as described in said patent, which disclosure is herein incorporated by reference, there is a selective production of non-filamentous micro-organisms capable of sorbing BOD under both anaerobic and oxidizing conditions and also capable of accumulating phosphates under oxidizing conditions. When the anaerobically treated mixed liquor is subsequently subjected to treatment with oxygen-containing gas under oxic conditions, the BOD therein is oxidized while phosphates are caused to accumulate in the solid biomass components. A portion of this activated biomass, separated from the oxidized liquor in the subsequent settling operation, is that which is recycled to the anaerobic treating zone.
To achieve the selective production of the desired non-filamentous micro-organisms, which in addition to sorption of BOD are effective in storing polyphosphates under oxidizing conditions (encountered in the subsequent oxic treating zone), the introduction of oxygen-containing gas to the anaerobic treating zone must be avoided; specifically, that zone must also be substantially free of oxides of nitrogen (i.e., less than about 0.3 ppm and preferably less than about 0.2 ppm NO.sub.x -- as elemental nitrogen) and the dissolved oxygen content of the mixed liquor in the anaerobic zone needs to be below 0.7 ppm and preferably below 0.4 ppm. Additionally, the introduction or presence of other oxidizing agents, e.g., ozone, peroxides, hydroperoxides, chromates, etc., is to be avoided.
The aforesaid '465 patent also describes a variation of the basic process wherein the mixed liquor is passed from the anaerobic zone to an anoxic zone positioned intermediate the anaerobic and oxic zones. The anoxic zone is defined as one receiving a portion of mixed liquor recycled from an oxic zone in which nitrogen oxides are present in concentrations higher than 2 ppm expressed as elemental nitrogen. As in the preceding anaerobic treating zone the dissolved oxygen concentration must be less than 0.7 ppm and the introduction of oxygen-containing gas and other oxidizing agents to the anoxic zone must be avoided. In this modification there is an internal recycle of mixed liquor from the oxic zone back to the anoxic zone, thus providing the nitrogen oxides (NO.sub.x) in the anoxic zone. In this modification, the intermediate anoxic treatment of the mixed liquor is employed for effecting denitrification.
Processes of the type described in said U.S. Pat. No. 4,056,465 employed in an activated sludge wastewater treating system an anaerobic mixing and treating zone followed by an oxic oxygenating or aerating zone, are identified as A/0.RTM. systems, whereas those systems also having an intermediate anoxic treating zone between the anaerobic and oxic zones are sometimes referred to as "A/A/O" or "A.sup.2 /O" systems.
U.S. Pat. No. 4,162,153 is directed to systems such as described in the companion '465 patent wherein a portion of the settled biomass from the secondary clarifier is recovered for use as a fertilizer or as a nutrient in animal feeds.
Further improvements in the operation of A/O and A.sup.2 /O systems are described in U.S. Pat. No. 4,271,026. According to this patent, more reliable and enhanced removal of phosphorus values from wastewater influent is obtained by maintaining operating conditions within the processing system encompassing the initial anaerobic treatment and extending through the process up to but not including the sludge separation step, a BOD to phosphorus (BOD/P) ratio from about 5:1 and up to about 50:1, wherein BOD is expressed in milligrams of soluble BOD.sub.5 (exclusive of that attributable to ammonia) per liter of influent, and P is soluble phosphate expressed in milligrams of elemental phosphorus per liter of wastewater influent. Also, the system is advocated to be operated at a food to biomass ratio (F/M) from about 0.09 to an upper limit of about 1.4, wherein F is the total weight of soluble BOD introduced into the process per 24 hour day and M is the weight of volatile suspended solids (VSS) in the process system.
It has been found in practice of the process of the '026 patent generally there is significant phosphate removal from the wastewater, but that frequently a reduction in the extent of phosphate removal has been observed even when operating within the guidelines set out in the patent. It was noted that excellent removal of phosphates was had during certain periods of the day and poor removal at others, all while operating within the teachings of the '026 patent. Studies revealed that the concentration of phosphate in the clarifier effluent was increased when the system was operating at high F/M ratio and decreased when operating at low F/M ratio. This situation is effectively remedied in accordance with the present invention by maintaining the minimum and maximum F/M within specified limits during a 24 hour operation period, as hereinafter set out. By operation within the defined limits excellent phosphate removal is achieved throughout the day without significant fluctuation.